Why is it so hard to move on from my ex?

we broke up in October the break up was really bad he went back to his ex told her all of my business and forced me to get a abortion We live together until our lease ends in December. after our breakup he asked me for a second chance 3 times the first 2 times he went back to his ex and the third time He changed his mind and said he needs to focus on himself and his goals and just wants us to be friends to make our living situation less awkward. i have tried to be the bigger person and be cordial with him even though i still have feelings for him. but every now and then he tries to have sex with me (which I refuse) and when i tell him i dont want to be friends he dont leave me alone until i change my mind. I’m having a hard time just being his friend I don’t know why I still have feelings for him after he treated me so badly usually if someone mistreats me i dont stick around How can I move on from him? my sister suggested that i go on dates but im not ready to meet anyone new yet and i dont want to waste anyone’s time
Why is it so hard to move on from my ex?
Post Opinion